Evaluation tests consist in measuring the flow of air exhaled/inhaled via a flowmeter connected to the mouth of the subject during particular manoeuvres indicated by a skilled operator.
There exist different types of flowmeters. The most common ones are the so-called Fleisch pneumotachograph, Lilly pneumotachograph, Pitot pneumotachograph, mass flowmeter, turbine flowmeter, ultrasound flowmeter and variable-orifice flowmeter.
The present invention regards in particular turbine flowmeters. FIG. 1 of the annexed drawings shows a patient wearing a nose clip A for closing the nostrils in such a way as to convey all the air exhaled into a flowmeter B provided with a grip D. Set between the flowmeter B and the mouth of the patient is an antibacterial filter C, of the disposable type, which is to prevent any contamination in the breathing-in phase. FIGS. 2 and 3 of the annexed drawings show in perspective view and side view, and at an enlarged scale, the antibacterial filter C according to the prior art, comprising a hollow body C1, substantially in the form of a circular disk, with an inlet portion C2 that is to receive a flow of air exhaled by the patient and an outlet portion C3 that is to convey the air that has traversed the filter C towards the flowmeter B. Interposed in the hollow disk-shaped body C1 is a disk of filtering material F (represented with a dashed line in FIG. 3).
FIGS. 4 and 5 are a perspective view and a cross-sectional view and at an enlarged scale of an example of turbine flowmeter according to the prior art.
The flowmeter B comprises a cylindrical tubular body B1, made for example of plastic material, in particular transparent plastic material, defining an inlet portion B2 and an outlet portion B3 for the flow of air that traverses the flowmeter B. Provided within the portions B2, B3 are two conveying devices B4, B5 (FIGS. 4 and 5) comprising a plurality of stationary fins, rigidly connected to the body of the filter B1 and shaped so as to impart a helical direction on the flow of air that traverses it. The structure of the two conveying devices B4, B5 is also used for supporting in a freely rotating way a shaft B6 carrying the blades (for example two blades at 180° or three blades at 120°) of a turbine rotor R. The rotor R, made of plastic material and of low weight, is set in rotation by the flow of air that traverses the flowmeter, after the flow has been converted into in a helical flow by the conveying device B4 at the inlet of the flowmeter.
The speed of rotation of the rotor R is detected by sensor means of any type, for example by means of a pair of photo-emitters Tx1, Tx2 and a pair of corresponding photo-detectors Rx1, Rx2 (see FIG. 6 of the annexed drawings) provided on the body of the flowmeter and designed to detect the interruption of the light beams emitted by the photo-emitters caused by the passage of the blades of the rotor R. The aforesaid optical detection device is consequently able to detect the speed of rotation of the rotor R, as well as also the direction of rotation. The signal at output from the photo-detectors is processed by electronic processing means and thus provides a reliable and accurate indication of the flow and of the volume of air emitted by the patient.
Spirometry is a consolidated technique in medicine. As regards the requirements of the necessary instrumentation, international standardization guidelines are available, amongst which the following may be cited:    ATS/ERS 2005: “STANDARDISATION OF LUNG FUNCTION TESTING” edited by V. Brusasco, R. Crapo and G. Viegi: “Standardisation of spirometry, European Respiratory Journal 2005; 26: 319-338;    ERS/ATS 1997: “Lung volume equipment and infection control”; European Respiratory Journal 1997; 10: 1928-1932.
One of the important requirements to be respected in apparatuses for spirometry is the protection of the airways of the patient from contact with viruses and bacteria that may be present in the instrumentation.
Said results can be achieved with the following methods:                1) use of instrumentation in which all the elements in contact with the air exhaled and inhaled by the patient are disposable (disposable flowmeter);        2) interposition of a disposable antibacterial/antiviral filter between the flowmeter and the mouth of the patient;        3) disinfection of all the parts in contact with the air exhaled and inhaled by the patient.        
Of the three methods listed above, the last one is not in general considered valid on account of the excessive costs and time involved.
As regards the first two solutions, i.e., use of a disposable flowmeter or adoption of the antibacterial filter, it is important to examine certain aspects in greater depth.
Disposable Flowmeter
The state of the art regarding disposable flowmeters offers different solutions already available on the market (see also the documents Nos. WO 2005/037102 A1, U.S. Pat. No. 5,419,326; U.S. Pat. No. 5,997,483). The main difficulties that are encountered in producing an entirely disposable flowmeter are the following:                it is difficult to keep the production cost low;        if the disposable flowmeter includes components that can alter the response of the flowmeter, it is necessary to calibrate the spirometer after each replacement of the flowmeter or to codify the flowmeter on the basis of the response given;        no other part of the spirometer must come into contact with the air exhaled/inhaled by the subject in so far as no protective barrier constituted by an antibacterial filter is present.Disposable Antibacterial Filter        
The use of a disposable antibacterial filter (for example, of the type illustrated in FIGS. 1-3 of the annexed drawings) is recommended by all standardization guidelines, where it is also necessary for the following requirements to be respected:                the expiratory resistance of the ensemble flowmeter plus filter must not exceed the limit of 1.5 cm H2O/l/s, up to flows of 14 l/s so as to guarantee that the results are not altered;        the connection between the filter and the flowmeter must be completely fluid-tight so that all the air exhaled by the patient is effectively measured;        the efficiency of the filtering barrier against the passage of viruses and bacteria must be adequate and demonstrated with tests conducted by independent bodies.        
Further considerations should moreover be added that do not commonly appear in the technical literature in this field, but that are equally important for the reliability of the measurements and the safety of the patient.                The response of any flowmeter can vary significantly according to the form of the antibacterial filter connected thereto (the geometry of the filter affects the characteristics of the air flow, with generation of possible turbulence).        There are available on the market low-cost and poor-quality disposable antibacterial filters, which have reduced filtering power and do not ensure the necessary performance.        At times it happens that an antibacterial filter is connected to a flowmeter having a diameter incompatible using adapter connectors that frequently introduce undesirable losses and increase the deadspace of the measuring system (i.e., the volume of air that the patient is forced to breathe again) in an unacceptable way and to the point of altering the fluid-dynamic characteristics for which the device is designed.        